1.Field of the Invention
The invention relates to an electropneumatic position regulator having a feed air port for connection to a compressed-air supply line, at least one exit air port for connection to a pneumatic drive of a servo unit, and a controllable valve group arranged in the electropneumatic position regulator.
2. Description of the Related Art
For the control of process valves, use is commonly made of pneumatic servo drives as servo units, in which a diaphragm that acts on a servo element, such as a lifting rod connected to a closing body, is acted on with a variable pressure, for the purpose of generating movements, via a gaseous medium that will hereinafter be referred to in general terms as “air”. The pressurization may occur on one side, where the diaphragm is subjected to a load on the other side by a spring, or on two sides, and where different pressures on the two sides of the diaphragm cause the deflection. The regulation of the one or more pressures is realized by an electropneumatic position regulator, in which at least one actuable valve, depending on the actuation thereof, distributes pressurized feed air via one or two exit air ports to one or both sides of the diaphragm in the servo drive.
DE 10 2004 048 689 B3 discloses a two-stage pilot-control valve for use in an electropneumatic position regulator. In a first stage, four flow resistances, of which at least one flow resistance is variable, are arranged in a bridge circuit. Two opposite circuit points in the bridge circuit are connected to feed air, and to exit air passing to the environment, respectively. Between the two other circuit points, a pressure difference is provided that is variable in magnitude and sign and which serves as a pneumatic output signal of the electropneumatic transducer, which is thus realized in the first stage, for actuation of the downstream second stage. In the second stage there is situated a valve group that has a control piston that can be operated by the pressure difference. The control piston serves to operate a pneumatic valve in the valve group, via which pneumatic valve, depending on the actuation thereof, either compressed air supplied via the feed air line is conducted to the exit air port to the servo drive, or air supplied via the same exit air port is discharged to the outside. The pneumatic drive of a regulating valve, for example, is connected to the exit air port. In the event of failure of the pneumatic energy (i.e., compressed air), it is often necessary for the drive actuated by the position regulator to automatically place the process valves into a safe position, i.e., a position that is safe in the surroundings of the respective process, generally “open” or “closed”.
It is increasingly also the case that, in the event of other faults in electropneumatic position regulators, within the context of functional safety, which is classified in safety requirement steps or by “Safety Integrity Level” (SIL), it is required that the predefined safe position be attained in as safe a manner as possible. Specifically, in a fault assessment, a distinction can be made between dangerous faults, in the case of which the safe position can no longer be assumed as a result of the fault or in response to a demand, and non-dangerous faults, in the case of which the appliance is duly no longer functional but is placed into the predefined safe position. It has hitherto commonly been possible for material wear in valves of the valve group, in particular wear of the seals or diaphragms commonly used in valves, to lead to dangerous faults in electropneumatic position regulators.
DE 42 39 431 A1 discloses a position regulator and a servo element having a position regulator of said type. For monitoring, if the servo element has a seal, in particular a slide seal, a leakage chamber is provided on that side of the seal which is remote from the pressure, which leakage chamber is connected to a pressure switch which, if a set pressure is exceeded, generates a signal output to the position regulator. As a seal, use is made in particular of the bushing that is commonly provided in regulating valves. By means of the pressure switch, is possible to monitor whether a pressure that lies above the admissible pressure is generated in the leakage chamber owing to leakage. The signal output may be used for generating an alarm signal or for performing a safety routine.